Adds a focused reference for the Docker-backed Matrix QA lane (CLI flags,
seven scenario profiles, eight env vars including the redaction toggle and
Tuwunel image override, scenario taxonomy, output artifact layout, and triage
tips). Source-of-truth checked against extensions/qa-matrix/src/cli.ts,
shared/live-transport-cli.ts, runners/contract/{runtime,scenario-catalog}.ts,
and substrate/harness.runtime.ts.
Registered in docs/docs.json alongside QA E2E automation.
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